When drilling a borehole in rock formations with a conventional tricone roller drill bit, the rate of penetration of the formations has been found to be proportional to the weight, or downward thrust, placed on the drill bit. However, when drilling through rock formations which lie at an acute angle to the longitudinal axis of the existing borehole, unequal resistance to the penetration by the drill bit causes the direction of the drilling to deviate from the existing borehole axis, with this deviation also being proportional to the weight on the drill bit. As there is normally a limit placed on acceptable deviations of the borehole axis, the thrust on the drill bit is backed off until an acceptably small deviation is attained. Of course, this results in a reduced penetration rate and higher drilling costs.
It has been known for some time that repetitive impact blows on a roller drill bit will increase the penetration rate of the drill bit and that, because of the short duration of each impact blow, any deviation of the borehole is minimized. Impact blows, therefore, can be used as a substitute for part of the weight on the drill bit.
The Temple-Ingersoll "Electric Air" percussive rock drill, which was employed in the early part of the twentieth century, comprised a hammer piston having first and second ends positioned in two separate air chambers, two compressor pistons with each compressor piston being connected to a respective one of the air chambers to form two closed air systems, a crankshaft which actuated the two compressor pistons at a 180.degree. phase difference, an electric motor for driving the crankshaft, and a drill bit threadedly connected to one end of the impact piston. however, all of this equipment, other than the drill bit, was located above the earth surface, and the drilling depths achievable by this equipment were very shallow.
Pneumatic downhole percussion drills, which have been employed for over twenty-five years in borehole drilling, use a gas to reciprocate a hammer piston so that the hammer piston delivers repetitive impact forces to an anvil surface on a roller drill bit, improving the penetration rate of the drill bit while at the same time minimizing the deviation of the borehole. Unfortunately, only about six percent of all boreholes drilled in rock formations are suitable for the use of air as the medium to flush drilling debris from the borehole during the drilling operation. Thus, drilling mud is employed as the flushing fluid in over ninety percent of all boreholes drilled in rock formations. consequently, the concept of extending the percussion advantage in air-flushed drilling to mud-flushed drilling has been an enduring goal in the borehole drilling industry.
One recent effort to employ a pneumatic percussion drill in a mud-flushed borehole is disclosed in Kennedy, U.S. Pat. No. 4,694,911, wherein an air actuated annular impact piston is contained in a drilling assembly having an axial mud flow path. This is accomplished by employing a special drill string having air intake and air exhaust passageways in the wall of each of the drill pipes in addition to the central mud passageway. The special drill pipe represents a substantial increase in cost, particularly in deep wells, as well as an added difficulty in assuring alignment of the air passageways from one drill pipe to the next drill pipe in the drill string.
Various attempts to develop a percussion drill for drilling mud-flushed boreholes utilizing the drilling mud as the only fluid supplied to the drill assembly have employed a direct mud drive approach. In the direct mud drive approach, the drilling mud is selectively directed to a first chamber containing the back end of a downhole piston to drive the piston downwardly to strike an anvil associated with the drill bit and thus impart an impact force to the drill bit, and then the drilling mud is selectively directed to a second chamber containing the front end of the piston to drive the piston back to the top of its stroke. The drilling mud exhausted from the piston chambers can then be utilized to flush debris from the drill bit and the borehole. A valve assembly or a combination of ports in a sliding element, either a sleeve or a piston, is used to switch the drilling mud flow from the back end to the front end of the piston and then from the front end to the back end of the piston in each impact cycle. One such direct mud drive is disclosed in Hall et al, U.S. Pat. No. 5,396,965.
There are several disadvantages to the direct mud drive approach that, collectively, have hindered the success of various attempts to date to commercially employ this approach in a mud operated impact drill. First, despite a filtering operation, the drilling mud generally contains some abrasive material such as sand, which causes erosion at the exposed edges and in the clearance spaces of the piston and the valves of the impact drill, resulting in a short operating life and high replacement costs. Second, the impact between the piston and the drill bit takes place in a mud bath, that is, each of the hammer end of the piston and the anvil surface on the drill bit is totally immersed in drilling mud prior to and at the point of impact. This means that a portion of the impact force is dissipated in squeezing mud out from between the hammer face and the anvil face prior to and at the moment of the impact. In addition, this high pressure squeezing can cause pitting to occur on the faces of the piston and the drill bit., again resulting in high replacement costs. Third, as the borehole becomes deeper, the back pressure against which the drilling mud must be exhausted, at the end of each piston stroke, increases. In turn, this reduces the pressure drop across the piston, which in turn reduces the impact force imparted to the drill bit, which in turn reduces the penetration rate of the drill bit. Fourth, as the pressure and flow rate of the drilling mud are dictated by borehole flushing requirements, the same pressure and flow rate may also be used to drive the piston. This does not provide any latitude to vary the energy or the frequency of the impact blows, as can be required by variations in the rock formations encountered in the borehole.